1. Field
The present disclosure relates generally to the field of thin-film sol-gel coatings and in particular to coating on substrates such as glass or solar panels.
2. Description of the Related Art
Thin-film sol-gel coating refers to a technique of coating substrates, such as optical surfaces, windows, solar panel surfaces, and the like, using a wet chemical formulation called a ‘sol’ that undergoes a ‘gelation’ process wherein it polymerizes to form a solid thin-film on a substrate. These thin-films often undergo a subsequent curing step to increase mechanical strength and other properties. This curing is often accomplished by heating or irradiating the substrate and coating. Thin-film sol-gel coating is a very versatile process that has many industrial uses such as formation of dielectric layers on semiconductor wafers and water repellent layers on ceramics. There are several well documented techniques for applying wet sol to substrates, some of which are in widespread industry use and others that have generally been limited to the laboratory. Industrial scale sol-gel coating is most commonly performed by a dip, spray, aerosol deposition, spin, meniscus, slot-die or roller process. There are also several methods used to cure sol-gel thin films including baking in ovens, treatment with microwave, infra-red or ultra-violet radiant energy, and exposure to flowing hot gases. These methods may or may not work in concert with components of the coating that catalyze or otherwise aid the curing process.
In the dip coating process the substrate to be coated is dipped into a tank containing the sol. It is then withdrawn at a process dependent speed. As the substrate is slowly drawn from the sol, the gelation process occurs just above the surface and a thin-film layer forms. Dip coating processes are inherently two sided in that all sides and edges of the substrate are coated. This can be advantageous if complete sol coverage is desired but is disadvantageous if the coating on some portion of the substrate interferes with a later substrate processing step. The dip coating technique requires a tank slightly larger than the substrate, which for large substrates means the tank may hold a large volume of sol. For sols mainly composed of organic solvents this may pose a vapor and flammability hazard. It may also be challenging to control the composition and quality of the sol within the large tank. Each new substrate dipped in the tank may carry contamination that is transferred to the sol; the sol might become depleted in some element as more substrates are processed causing a variation in the thin-film produced. The sol may change through evaporation of solvent at the surface where substrates are introduced.
Spray coating exists in many forms, but generally may be considered to be the deposition of material through a nozzle under pressure or the atomization of material which is then entrained by a jet of gas. In all cases the material is moved across a gap between a nozzle and a surface to be coated. The purpose of the spray system is to deposit a uniform layer of material over a wide area of the substrate. In the context of sol-gel coatings on substrates spray coating has the advantage of only applying fresh material to the substrate. Careful selection of solvents and control of solvent evaporation is needed to ensure that the correct final concentration of sol is delivered to the substrate. Spraying typically requires that either the nozzle or the substrate is moved in order to coat an area, for example the substrate may be moved past a line of stationary nozzles.
Spin coating is commonly used in the semiconductor wafer processing industry and in the LCD display panel industry to apply even layers of material to the surface of flat substrates such as silicon wafers or large pieces of glass. It has the same advantage as spray coating in that only fresh material is deposited. It also has excellent uniformity control. Generally, equipment to perform the spin coating tends to be complex and costly to maintain because of the fine mechanical control needed to achieve uniformity. This is particularly true as the size of the substrate increases.
Meniscus coating was historically used in the semiconductor industry before giving way to spin coating. It remains in use by some equipment vendors in the LCD display industry. Meniscus coating works by passing a substrate to be coated over a narrow slot at a very close distance such that material forced up through the slot forms a continuous meniscus with the substrate. As the substrate moves across the slot this meniscus deposits a layer of material on the substrate. The technique requires fine control over the distance between the slot and the substrate across the full length of the slot. Generally, the substrate must be extremely flat to avoid deviation in this distance. Additionally, this technique works best with viscous materials that can form a large meniscus. This limits its usability with sol-gel formulations that use comparatively low viscosity solvents.
Roll coating is a common application method for sol-gel coatings on flat substrates. In one embodiment of this process, material is deposited from a reservoir onto an application roller. A doctor blade or doctor roller may be used to control the thickness of the coating material placed on the application roller. That material is then transferred directly from the application roller to the substrate. In general, roll coating works best with continuous substrates, such as, for example, a roll of steel. In the case of discontinuous substrates such as pieces of glass or wood, for example, special techniques may be employed to control coating uniformity at the leading and trailing edges of the substrate. These techniques include, for example, varying the application roller contact pressure by having the coating roller touch-down on the leading edge and lift-off the trailing edge in a precisely controlled manner. The application roller may run in a forward direction, i.e. rolling with the substrate direction of movement or in a reverse direction, wherein the application roller opposes the direction of movement of the substrate. The surface of the application roller may be made of a compliant material that serves to compensate for any surface or flatness variations on the substrate and to provide a surface to which the coating material will adhere in a reasonably uniform manner, or the application roller may be a comparatively solid material. Depending on the rheology of the material to be coated, the surfaces of the rollers may be patterned with grooves or other textures to add in coating application.
Flow coating is a technique where coating material is flowed over a surface to be coated. The excess drips away and that which remains on the surface forms the final coating. The surface may be flat or irregular. In general, the substrate is oriented such that the coating material flows due to gravity. Advantages of this technique are its simplicity, ability to coat irregular surfaces, and the option to use only fresh material or to recirculate the excess material that drips off the surface.
It would also be preferable to enable drying and curing of such coatings at relatively low temperatures, such as below 150° C. so that the coatings could be applied and dried and cured on substrates to which other temperature sensitive materials had been previously attached, for example a fully assembled solar panel.
The curing process for sol-gel films is a separate process that occurs after the gelation of the sol-gel material. One common cure method is to heat a sol-gel coated article in an oven. This has the advantage of simplicity. The oven may be of the batch type wherein a batch of coated material is placed in an oven that is then sealed, and maintained for a period, then opened and the batch removed. While in the oven, the coater material may be subject to a varying temperature profile created by the oven's controller. Alternatively, the oven may be of the continuous type wherein a conveyor belt or similar transport mechanism moves coated articles through a heated container. As the material moves through the container it may experience different temperatures in different zones creating a temperature profile consisting of heating, soaking at a fixed temperature, then cooling. The profile may be a function of the temperature zones within the oven and the speed of the transport mechanism. Heat within the oven may be provided by convection with hot gases created by combustion of fuel gas or by the heating of gas by electrical elements. Alternatively, the coated article might be heated by radiant heat.
Some types of sol-gel coatings may be cured with ultra-violet radiation. In these types of materials, chemical crosslinking within the material is promoted by high-energy photons.
For the curing of thin coatings on surfaces, hot gasses may be passed directly over the thin-film to heat the surface layer by conduction.
Optimal methods for industrial scale sol-gel coating of flat substrates should be capable of selectively coating just one face of a substrate; be economical in their use of the coating material; provide easy compositional and contamination control; be versatile with respect to the sol-gel formulation such that solvents of different volatilities can be used and chemically compatible with critical equipment; be of low complexity and cost; capable of handling large imperfections in substrate surface flatness, and capable of achieving superior coating uniformity. Optimal curing methods should be cost effective; not damage the coated substrate; match the through-put of the prior coating process step and effectively cure the coating material to its final desired properties.